In recent years, due to increasing transmission traffic, there is a demand to increase the capacity of an optical transmission system. With respect to a short-distance transmission system, a low-cost, simple configuration is also demanded, in addition to a larger capacity.
Accordingly, application of a discrete multi-tone (DMT) modulation scheme to an optical transmission system is being considered. The DMT modulation scheme is one of multi-carrier transmission technologies based on an orthogonal frequency division multiplexing (OFDM) technology, and is used in a digital subscriber line (DSL) technology such as an asymmetric DSL (ADSL). Therefore, the DMT modulation scheme is referred to also as a “multi-carrier modulation scheme”.
The DMT modulation scheme uses a technique called “bit loading”, which allocates, to each subcarrier, number of bits according to transmission characteristics. The transmission characteristics may be referred to as “receiving characteristics” or “signal quality”.
Example indices of transmission characteristics include optical signal-to-noise ratio (hereinafter referred to as “OSNR”) and a bit error rate (BER). The transmission characteristics may be referred to also as a “transmission condition”. For example, the DMT modulation scheme allocates a larger number of bits to a subcarrier with high transmission characteristics than to a subcarrier with low transmission characteristics. The DMT modulation scheme may thus increase a frequency use efficiency of a transmission band, and increase a transmission capacity.
Accordingly, by installing the DMT modulation scheme in an optical transmission system, the frequency use efficiency (i.e., transmission capacity) is increased, and miniaturization and reduction in cost may be achieved.
Patent Document 1: Japanese Laid-open Patent Publication No. 2014-107854
The DMT modulation scheme considers use of one or more subcarriers for realization of functions instead of for data transmission. The functions include a link communication channel (LCC). The LCC is used, at the time of activation of a system, for transmission of a control command (control signal) between an optical transmitter of a first optical transmission device and an optical receiver of a second optical transmission device, so as to reliably realize data transmission between the first optical transmission device and the second optical transmission device.
However, with the DMT modulation scheme, a subcarrier with a significantly deteriorated OSNR, i.e., transmission characteristics, is sometimes present. For example, such a dip in the transmission characteristics is caused by an influence of power fading which is due to a frequency chirp added in a transmission optical signal depending on frequency characteristics of a light source of the optical transmitter, and to dispersion to which the transmission optical signal is subjected in an optical transmission line.
In the case where there is a subcarrier with a dip in the transmission characteristics, it is not desirable to allocate the LCC to the subcarrier where a dip is caused in the transmission characteristics. If the LCC is allocated to the subcarrier where a dip is caused in the transmission characteristics, the OSNR may be significantly reduced, and the LCC is possibly not established. If the LCC is not established, activation of the system is prevented.
On the other hand, it is also not desirable to allocate the LCC to a subcarrier with very good transmission characteristics. A subcarrier which is allocated with the LCC is used to activate the system, and is not used for data transmission. That is, if the LCC is allocated to a subcarrier with very good transmission characteristics, transmission capacity is reduced or reception sensitivity is reduced, thereby affecting the transmission characteristics.